Effects Of Angular Orientation On Piloted Ignition And Lateral Flame Spread Of Typical Solid Combustibles

The initial stage of fire is determined by the process of pyrolysis and ignition of combustible solids, and the developmental stage of fire is determined by the process of flame spread. Both of the two aspects are the key topics in fire science, and have been attracting the attention of many scholars. Under the external radiation, the temperature of combustible solids will keep rising, and ultimately decompose and release small molecules pyrolysis gas when reaching a certain temperature. The pyrolysis gas will continuously separate fromthe solid surface and get mixed with the ambient air, forming premixed combustible gas. When the concentration of combustible gas and the temperature reaches a critical condition, the gas phase chemical heat release will irreversibly outstripthe convective heat loss, and the local gas phase temperature gets sudden rise, formingthe macroscopic ignition. The surface heat flux and the surface temperature will subsequently get sudden rise due to the heat transfer from the flame, thus the fuel will produce more pyrolyzate to participate in the gas phase combustion reaction with increasing combustion intensity until reaching thermal equilibrium state. The heat transfer takes place from the flame to burned zone, and simultaneously from the flame to unburned zone, thus enhancing the temperature of the unburned zone, accelerating the pyrolysis rate and furtherproducing more pyrolyzate to sustain the combustion. This phenomenon is manifested as movement of pyrolysis front on the macro, i.e. the spread of the flame.Igniton and flame spread of combustible solids are essentially related to the pyrolysis gas flow over the solid surface. When the sample orientation changes, the pyrolysis gas flow field gets changed, which eventually leads to the change of the ignition and flame spread behaviors. However, previous researchers have paid little attention to this phenomenon, and most of the ignition studies have focused ont the horizontal samples, neglecting other orientations samples. For the study of flame spread, most cases have beenabout horizontal, upward (including inclined upward) and downward (including inclined downward). Almost no scholars have studied lateral flame spread over inclined surface. Based on the weakness of previous researches, this thesis presents experimental and theoretical study on both pyrolysis-ignition process and flame spread process. Details are as follows.For pyrolysis and ignition, the conical heater was utilized to test the pyrolysis-ignition characteristics of paulownia wood in three orientations (upward, vertical and downward) under a series of radiant heat fluxes. The experiments verified the orientation effects on pyrolysis and ignition process. Meanwhile, we established one-dimensional pyrolysis model of wood with boundary condition of the convection affected by orientation, and then investigated the surface temperature in the numerical way. The calculated results suggested that pyrolysis and ignition process was affected by the orientation in two ways:convective heat transfer and radiation blockage by pyrolysis gas.For flame spread, experiments of lateral flame spread over inclined PMMA surface wereconducted with the rotary apparatus. Based on the assumption of dominated heat transfer mode of convection, the theoretical model of lateral flame spread wasdeduced. It waspredicted by the model and confirmed with experimental results that flame spread rate would increase with the inclination angle but decrease with the sample width. Further, research of flame appearance in flame spread process wasmade. It is concluded that pyrolysis length follows a power-law increase with the sample width and angle sine and flame length follows a power-law increase with the sample width aloneMoreover, experiments of burning behaviors of inclined PMMA were conducted in Lhasa and Hefei, including steady burning state and flame spread state. A model for steady burning state is established and it suggests that burning rate increases with ambient pressure and inclination angle corresponding to experimental results. In low pressure environment, the gas phase chemical reaction rate would have a significant influence on flame spread rate.